The invention relates to aluminium alloy strips with high surface homogeneity, intended for applications requiring a good visual quality or specific optical properties, such as reflectors or anodised plates for construction and decoration. It also relates to a method for making said strips by twin-roll casting.
Twin-roll casting has been widely used for several decades to manufacture foils or standard sheets made of aluminium alloys. As described in the basic patent FR 1198006 filed in 1958 by Pechiney, it consists of introducing the liquid metal, stored in a supply tank, in the gap between two cooled horizontal rolls rotating in opposite directions, using an injector. The metal is solidified in the form of a continuous strip, while being reduced in thickness due to the roll pressure. These continuous casting machines are very often used to produce strips between 5 and 12 mm in thickness. In their most recent versions, such as Pechiney Rhenalu""s JUMBO 3 CM(copyright) casting, they also make it possible to cast thinner strips less than 5 mm thick as described, for example, in the patent FR 2737430.
The strips from these machines are very rarely used as cast. They generally undergo a first cold rolling sequence and, for some applications, a second finishing cold rolling sequence, possibly with special rolls.
These usual twin-roll casting machines make it possible to obtain strips of a homogeneous appearance, but for very demanding applications in terms of surface condition, associated with a surface treatment of the strip liable to reveal existing surface defects or create said defects from metallurgical heterogeneities, e.g. an anodic treatment, chemical or electrolytic brightening, pickling, chemical brightening, cataphoresis or enamelling, the surface quality of strips obtained from twin-roll casting is currently not sufficient. The upper side of the cast strip generally comprises ripples, presented in the form of lines perpendicular to the direction of casting (transverse-longitudinal direction), which appear to be caused by the oscillation of the meniscus of liquid metal during casting. After anodic treatment, these ripples become visible in the form of parallel streaks; this is a visual defect which is conveyed by a difference in the grey levels with a pitch of the order of one to several (e.g. ten) millimeters.
Another surface defect frequently observed consists of parallel mechanical scratches along the strip; this is a roughness defect. The lower side is of usual mill finish quality.
In addition to these two types of surface defect, accidental scratching which is not specific to the twin-roll casting technique occasionally occurs.
The need to improve the surface appearance of strips obtained by continuous casting has been expressed for some time and a number of solutions have been proposed.
For example, American patent U.S. Pat. No. 4,461,152 describes a liquid metal treatment process starting with the injection in the liquid metal of a gas containing chlorine, followed by the passage of the liquid metal through a series of coalescence chambers, and completed with a filtration, thus enabling the reduction of the inclusion rate in the liquid metal, resulting in the improvement of the surface appearance of 5086 and 5182 alloy strips for computer disks. However, manufacturers now tend to try to minimise the use of chlorinated gases.
German patent application DE 2443068 of 1974 describes a continuous casting machine between steel belts which aims to improve the surface appearance of aluminium or aluminium alloy strips so as to be able to produce strips for a decorative anodic treatment. The technical solution proposed in said patent application cannot be applied to twin-roll casting, for three reasons:
The surface quality of a strip obtained by continuous casting between belts is intrinsically poorer than with twin-roll casting, which is probably due to belt vibrations. The metal solidification conditions are totally different, since, for continuous casting between belts, the centre of the strip is solidified downstream from the level of the axes of the rolls near the injector, while, for twin-roll casting, it is solidified upstream from this axis. Finally, continuous casting between belts cannot be used to obtain thin strips less than 5 mm thick; consequently, the gap between the rolls is smaller in a twin-roll casting machine.
British patent application GB 2198976 describes an asymmetrical injector device making it possible to increase the casting rate and, therefore, the industrial output of a twin-roll casting machine; the document does not describe an improvement in the surface of the products obtained, which was not the subject of the invention described.
American patent U.S. Pat. No. 5,350,010 aims to optimise the surface quality of strips intended to produce offset printing plates by the precise monitoring of the grain size of the final product, which implies that a certain metal composition is complied with and that certain process parameters, occurring downstream from the continuous casting, such as the cold rolling pass reduction rate, are monitored. Given that the surface defects present on the as cast strip do not generally disappear during the cold rolling operations that follow, this approach, in the applicant""s opinion, does not seem to deal with the cause of the defects but only tries to minimise their consequences on the finished product.
Following a similar technical approach, patent application EP 0821074 also describes a conversion process of a strip obtained by twin-roll casting, enabling the manufacture of offset plates.
The applicant has noted that none of these approaches make it possible to obtain surfaces meeting high optical and visual homogeneity requirements directly by twin-roll casting, possibly followed by several cold rolling passes.
The purpose of the present invention is to obtain, by twin-roll casting, aluminium alloy strips with, on at least one side, high surface homogeneity and that can be used for applications that were previously not open to them. It should be noted that the majority of these applications only absolutely require a very homogeneous surface condition for one of the two sides. Therefore, it is not a problem that the present invention only produces a spectacular improvement in the surface condition for one of the two sides.
The strips according to the invention, cast continuously between two rolls, less than 12 mm thick, preferably less than 5 mm, comprise an upper side, the surface condition of which may be characterised at three different stages of manufacture, corresponding to three types of more or less elaborate industrial products, on specimens subjected to a particular preparation, representing a typical industrial surface treatment revealing surface defects:
a) The as cast strips show on the surface (excluding accidental mechanical scratching visible to the naked eye) of their upper side, after creating a 1 xcexcm thick anodic oxide layer by sulphur anodic treatment, an optical roughness value SN, measured on three 5 cm longitudinal sections and three 5 cm transverse sections, such that its mean variation on each section, defined by the ratio:
(Maximum SNxe2x88x92minimum SN)/Mean SN 
xe2x80x83is less than 20%, and the difference: xcex94SN=SN maxxe2x88x92SN min is less than 20.
b) The strips after cold rolling to a thickness between 4 and 0.1 mm, preferably between 2 and 0.1 mm, show on the surface of their upper side an optical roughness value SN, measured under the same conditions, on a specimen having undergone an alkaline pickling treatment on 10 xcexcm followed by a sulphur anodic treatment resulting in the creation of a 1 xcexcm thick anodic oxide layer, such that its mean variation on each section is less than 20% and xcex94SN is less than 12.
c) The strips that have undergone a first cold rolling followed by a finishing rolling to a roughness Ra less than 0.2 xcexcm and brightened electrolytically show on the surface of their upper side, after creating a 1 xcexcm thick anodic oxide layer by sulphur anodic treatment, an optical roughness value SN such that its mean variation on each section is less than 20% and xcex94SN is less than 3.5 and even 0.5.
The strips according to the invention also have a surface homogeneity such that the 2D roughness range distribution asymmetry value (Sk or skewness parameter), measured using an optical scanner-based technique described below, is between xe2x88x920.2 and +0.3, and preferentially between xe2x88x920.1 and +0.2. The 3D roughness (Ek parameter), determined by a mechanical sensor parallel to the direction of rolling according to a method described below, is less than 15, and preferably 8.
The invention also relates to a method for making aluminium alloy strips by continuous casting between two cooled rolls, from a casting tank containing the liquid metal connected to an injector, comprising an upper lip and a lower lip, feeding the liquid metal into the gap between the two rolls, wherein the upper lip of the injector is recessed by at least 2 mm, and preferably by at least 5 mm, with reference to the lower lip.
The level of liquid metal in the casting tank, measured from the median casting level, is kept below 30 mm and preferably below 25 mm.